Movable switch contact assembly with self-aligning bridging plate members



June 29, 1965 G. A. wlLsoN, JR 3,192,328

MOVABLE SWITCH CONTACT ASSEMBLY WITH SELF-ALIGNING BRIDGING PLATE MEMBERS Filed Feb. 1, 1965 2 Sheets-Sheet 1 flql/ f@ INVENTOR.

3,192,328 BLY WITH SELF-ALIGNING June 29, 1965 v G. A. wlLsoN, JR

MOVABLE SWITCH CONTACT ASSEM BRIDGING PLATE MEMBERS 2 Sheets-Sheet 2 Filed Feb. 1, 1963 Lily; HHH" INVENTOR.' i ZZ/Zsomd'rz I Gordan ttomey United States Patent C) MOVABLE SWITCH CONTACT ASSEMBLY WITH SELF-ALIGNING ERIDGING PLATE MEMBERS Gordon A. Wilson, Jr., Washington, Pa., assigner to McGraw-Edison Qompany, Milwaukee, Wis., a corporation f Delaware Filed Feb. 1, 1963, Ser. No. 255,615 9 Claims. (Cl. 20G- 11) This invention relates to multiple position rotary elec'- tric switches .and in particular to movable contacts which bridge stationary contacts of multiple position rotary switches.

It is frequently desirable to adjust the voltage ratio of an electrical transformer. In the conventional arrangement for adjusting transformer voltage ratio, taps are provided in one of the transformer windings and a multiple position, rotary tap changer switch having stationary contacts connected to the taps is operated to increase or decrease the number of active turns in the tapp-ed winding with respect to another winding of the transformer. In one known arrangement the tap changer switch stationary contacts are uniformly spaced on the circumference ofa circle, and Iadjacent stationary contacts are bridged on their upper and lower surfaces by a movable contact including upper and lower conductive plates which are resiliently urged into high pressure engagem-ent with the stationary contacts. In tap changer switches which carry large currents and may carry heavy short circuits, the upper and lower bridging plates of the movable contact assembly may be constructed to have three point support and to be self-aligning in order to compensate for wear and misalignment lof the stationary contacts, thus tending to prevent improper engagement with the stationary contacts and facilitating high contact pressure therewith. When such tap changer switches carry fault current, overheating occasionally occurs as a result of high contact resistance between one of the stationary contacts and one of the bridging plates. The magnitudes ofcurrent that the movable contact can carry without overheating limits the k.v.a. rating of prior a-rt tap changer switches. Further, the high forces imparted to the bridging plates by the magnetic attraction therebetween, when fault current flows in parallel paths through the plates, were transmitted directly to the driving arm of prior art tap changer switches. It is an object of the invention to provide a movable contact assembly for a multiple position .tap changer switch which can carry substantially higher magnitudes of current than prior art constructions. It is a further object of the invention to provide such a tap changer switch movable contact assembly wherein, it a high resistance exists between a stationary contact and one bridging plate, the resulting unbalance current can transfer to the other bridging plate and thus prevent over heating. Another object of the invention is to provide such a tap changer switch movable contact assembly wherein magnetic forces imparted to the bridging plates when high fault current ows therethrough are not transmitted to the radial driving arm of the tap changer switch. Still another object of the invention is to provide such a tap changer switch movable conatct assembly wherein the self-aligning bridging plates have greater freedom of movement than prior art constructions.

These and other objects and advantages of the invention will be more readily apparent from the 'following detailed description of the invention when taken in conjunction with the accompanying drawing wherein:

FIG. l is a partial front View of a multiple position tap changer switch illustrating an embodiment of the invention;

3,192,328 Patented .lune 29, 1965 ice FIG. 2 is a View taken along lines 2 2 of FIG. l;

FIG. 3 is a view taken along lines 3 3 of FIG. 2;

FIG. 4 is a partial front view of a multiple position tap changer switch illustrating another embodiment of the invention;

FIG. 5 is a view taken along lines 5-5 of FIG. 4;

FIG. 6 is a view taken along lines 6-6 of FIG. 5 and FIG. 7 is a view taken along lines 7-7 of FIG. 4.

The multiple position -tap changer switch shown in FIGS. 1 has a' plurality of stationary contacts 10 uniformly spaced apart about the circumference of a circle and fastened on an insulating panel 11. Each stationary contact 10 is of suitable conductive metal such as copper and preferably is a single piece and includes a blade portion 12 integral with :a shoulder portion 14 and a terminal stud portion 15 which extends through an insulating tube 16 positioned in an aperture 17 in insulating panel 11. Insulating collars 13 and 19 surround insulating tube 16 on opposite sides lof panel 11, and a nut 20 threaded on terminal stud portion 15 clamps insulating collar 18 between shoulder portion 14 and panel 11 and insulating collar 19 between panel 11 and a washer 21 to aiiix stationary contact 16 to panel 11. Pin portions 22 depending fnom shoulder portion 14 fit into apertures in insulating collar 18, and pins 23 inserted in registering apertures in collar 13 and panel 11 prevent stationary contact 1t) from rotating about the axis of terminal stud portion 15.

The blade portion 12`has generally flat upper and lower contact surfaces. The blade portions 12 of all stationary contacts 1t) are in a plane perpendicular to the axis of the circle about which they are arranged and are spaced substantially uniformly from insulating panel 11. A rotatable drive shaft 24 extends along the axis of the circle about which the stationary contacts 10 are arranged and is journalled within an aperture in panel 11. Shaft 24 carries a radially extending rotatable driving arm 25 which is in the medial plane of the blade portions 12 or the stationary contacts 10. An insulating tube 26 surrounds shaft 24. Driving arm 25 is generally flat and is integral with a tubular split clamp 27 which surrounds insulating tube 26. Clamp 27 is affixed to shaft 24 by a nut 2S threaded on a bolt 29 which extends through clearance apertures in portions 30 at the split end of clamp 27 and also by a pin 31 extending transversely through the split clamp 27, insulating tube 26 and drive shaft 24 to positively prevent movement of clamp 27 and arm 25 axially of shaft 24.

An aperture 35 extends through fiat arm 2S generally parallel to the -axis of shaft 24. A pair of generally flat, bridging plates 34 of suitable conductive metal such as 4copper are disposed on opposite sides of arm 25, and thus on opposite sides of the median plane of the stationary contacts 1i). Each conductive plate 34 is of .sutlicient width to bridge between the blade portions 12 of adjacent stationary contacts 1t). The bridging plates 34 are preferably identical and each carries a pair of circumferentially space-d apart spherical contact buttons, or-protruberances 36. The circumferential spacing between contact buttons 36 on each plate 34 subtends approximately the same yangle with the axis of shaft 24 as the angle subtended by adjacent stationary contacts 10. Each bridging plate 34 at the radially inner end has an axially extending portion 37 which protrudes into the aperture 33 in driving arm 25 and engages a correspond-ing axially ending portion 37 on the other bridging plate 34. The abutting ends 38 of the axially extending portions 37 are preferably arcuate and meet in point contact to facilitate rocking movement of one bridging plate 3-4 relative to the other and to assure self-alignment of the bridging plates 34. It will be appre- 53 ciated that each bridging plate has three point support on the .two pr-otuberances 3e `and the axially extending portion 37 so that it is free to move or rock as it is rotated by arm 25 and that, consequently, it is almost impossible for it to improperly engage the stationary contacts 1t).

A pair of circumferentially spaced apart clearance apertures 40 are provided in each bridging plate 34, and each aperture 40 is preferably on a line which forms one of the sid-es of the triangle defined by the three points of support of the bridging plate 34. The apertures 4d in the upper and lower bridging plates 34 are in register with each other and also with a pair of circumferentially spaced apart clearance apertures 41 in driving arm 25. Means for providing a driving connection between arm 25 and the bridging plates 34 and for resiliently urging the bridging plates 34 toward each other and into high pressure engagement with opposite sides of blade portions 12 while permitting vrocking movement of bridging plates 34 independently of arm 25 include a pair of headed driving pins 44 extending through the registering apertures 4l and 40 in arm 25 and bridging plates 34 respectively and -c-ompression springs 45 cir-cumjacent pins 44 between the head 48 on 'the pins 44 and one bridging plate 34 and also be tween the other -bridging plate 34 and cup-shaped spring caps 46 fitting over the ends of the springs 45 and the driving pins 44 and secured thereto by through pins 47 which extend transversely through apertures adjacent the end of pins 44. The pins 44 preferably have a clearance tit in the apertures 4i? and 41 so that the bridging plates 34 are in loose engagement with the driving pins 44 and the driving pins 44 are in loose engagement with arm 25 with the result that the bridging plates 34 are not restrained from tilting and rocking relative to the arm 25. The springs 45 urge the spherical contacts 36.011 bridging plates 3-4 into firm and positive contact with the blade portions 12 of the stationary contacts Iii. The spacing between the upper and lower bridging plates `34 is limited to a predetermined distance bythe flat arm 25.

It will be appreciated that the bridging plates 34 having axially extending portions 37 `protruding into .aperture 33 in arm 25 will have greater freedom of lmovement to tilt and rock than prior art devices wherein the radially inner end of the bridging plates rested and had point support on the driving arm. Normally each bridging plate 34 will carry half of the current, but if a high contact resistance yoccurs between a blade portion l2 and a protuberance 36 on one bridging plate 34, the current can transfer -to the other bridging plate 34 through the electrically engaged axially vextending portions 37, thereby preventing undesirable heating resulting iiow of current through such high resistance. Further7 since the axially extending portions 37 protrude through aperture 33, rather than resting upon the driving arm as in prior art devices, any mechanical `forces imparted to the bridging plates 34 as a `resul-t of the large magnetic forces caused by fault currents owing in parallel paths through the bridging plates 34 are not transmitted to the arm 25.

The embodiment of multiple position, tap changer switch movable contact illustrated in FIGS. 47 has all the advantages of the embodiment of FIGS. l-3 and additionally can carry much higher magnitudes of fault current. Elements of the movable contact assembly shown in FIGS. 4-7 similar to those of the embodiment of FIGS. 1-3 Iare given the same reference numerals with the addition of the prime designation, and description thereof will not be repeated. The tap changer switch movable contact assembly includes a pair of first, or inner, bridging plates 34', similar to the bridging plates 34 of the embodiment of FIG. 1-3, and also includes a pair 'of second, or outer, bridging plates 5t) of suitable conductive metal such `as copper disposed Von opposite sides of arm 25 and of the median plane of the stationary contacts and superimposed over the inner bridging plates 34. Bridging plates 34 carry a pair of circumferentially spaced apart spherical contact buttons 36, and the circumferential lea,

spacing between protuberance Se subtends a slightly greater angle with the axis of drive shaft 24 than the angle therewith subtended by radial lines through the center of blade portions 12 of adjacent stationary contacts 10. Each outer bridging plate 5@ carries a pair of relatively elongated silver contact buttons '52 which protrud-e through clearance apertures 53 in inner bridging plates 34', and the circumferential spacing between silver contact buttons 52 subtends a lsmaller angle with the axis of drive shaft 24 than the angle therewith subtended by radial lines through the center of :adjacent blade por-tions 12. In other words, the circumferential spacing between elongated contact buttons 52 on outer bridging plates `5t) is less than that between protuberances 36 ion inner bridging plates 34. Each inner bridging plate 34 has anaxially extending portion 37 protruding into aperture 33' in arm 25' and engaging a similar axially extending portion 37 on the other inner bridging plate 34.- Each outer bridging plate has an axially extending portion 55 protruding into aperture 33' and engaging asimilar axial-ly extending portion in the other outer bridging plate 50. The abutting ends 56 of the axially extending porti-ons 55 are preferably arcuate to provide point Contact and assure self-alignment of the outer bridging plates 5t).

The clearance apertures 4d in the inner bridging members 34 and the driving pins 44 extending therethrough and through the clearance apertures 41 in arm 25 are circumferentially spaced a greater distance than in the 'embodiment of FIGS. 1-3 so that the driving pins 44' are beyond the outer bridging plates 5b but still provided a driving connection between arm 25 and inner bridging plates 34 and permit the inner plates 34 to tilt and rock independently of driving arm y25. Compression springs 45 circumjacent driving pins 44 resiliently urge the inner bridging plates 34 into high pressure Contact with stationary blade portions 12.

A pair of circumferentially spaced apart clearance holes 6@ in each outer bridging plate 50 are in register with the corresponding clearance holes 60 in the other outer plate 5i) and also with clearance holes 61 in both inner bridging members 34 and with similar clearance holes 62 in arm 25'. Means for providing a driving connection arm 25 and outer bridging plates 50 and for resiliently urging outer bridging plates 5) into engagement with the blade portions 12 of the stationary contacts l0 while permitting rocking and tilting of outer bridging plates 50 relative to arm 25 and relative to inner bridging members 34' includes a pair of headed driving pins 64 extending through the registering apertures 60, 61, and 62, compression springs 65 circumjacent driving pins 64 between the lower outer bridging plate 50 and the head on pins 64 and also between the upper outer bridging plate 50 and cup-'shaped spring caps 66 fitting over the ends of the springs 65 and the driving pins 64 and secured to driving pins 64 by through pins 67 extending transversely through holes ad- 'jacent the ends of driving 4pins 64.

Each outer bridging plate 50 is free to move and rock independently of arm 25' and inner bridging plates 34' as the plates 5t) are rotated by driving arm 25', and since each outer bridging plate 5t) has three point support on the relatively elongated silver contact buttons 52 and the axially extending portion 55, it is substantially impossible for it to improperly engage the blade portions 12' of the stationary contacts l10. The floating, self-alignment of the inner and outer bridging plates 34 and 50 causes each contact button thereon to bear with equal, high contactY pressure on the blade portions 12 of adjacent stationary contacts 10.

The lr.y.a. rating of the tap changer movable contact assembly shown in FIGS. 4-7 is substantially double that of the embodiment of FIGS. 1-3 since both the inner bridging plates 34 and the outer bridging plates 50 are self-aligning and are urged into high contact pressure engagement with the blade portions 12 of adjacent station ary contacts 10'. Each inner bridging plate 34 and outer bridging plate 5t) can carry the same current without overheating as a bridging plate 34 of the embodiment of FIGS. l-3. The high mechanical forces imparted to the outer bridging plates 50, resulting from magnetic attraction therebetween when short circuit currents flow in parallel paths therethrough, are not transmitted to the driving arm 25 since the third support point is not on the driving arm as in prior art construction.

While only two embodiments of the invention have been illustrated and described, many modifications and variations thereof will be readily apparent to those skilled in the art, and consequently it is intended to cover in the appended claims all such modifications and variations which fall within the true spirit and scope of the invention. For example, the driving pins 64 for the outer bridging plates dit in the embodiment of FIGS. 4-7 can extend through a single aperture in each inner plate 34'; the outer bridging plates 59 can be wider and subtend greater angles with shaft 24 than the inner bridging plates 34 and the driving pins e4 for such inner bridging plates 34 can extend through clearance apertures in the outer bridging plates 50 and still permit independent, self-aligning movement of the inner and outer bridging plates; or a single connecting bolt extending through the driving arm and each of a pair of bridging members may constitute the sole means for providing a driving connection between the driving arm and such pair of bridging plates and means may be provided to prevent turning of the bridging plates about the axis of such connecting bolt.

I claim:

1. In combination, a plurality of stationary contacts uniformly spaced in an arc about an axis, an operating shaft extending along said axis, a driving arm extending radially from said shaft and having an aperture therethrough, a pair of conductive, inner bridging plates on opposite sides of the medial piane of said stationary contacts and of said arm, a pair of conductive, outer bridging plates on opposite sides of said arm and superposed over said inner bridging plates, each bridging plate having a width greater than the space between adjacent stationary contacts to bridge said adjacent stationary contacts, each bridging plate having a portion extending into said aperture in said driving arm and said portions on said inner plates being in engagement and said portions on said outer plates being in engagement, whereby current can transfer from one of a pair of bridging plates to the other if a high resistance exists between said one bridging plate and a stationary contact, means for providing a driving connection between said arm and said inner bridging plates and for resiliently urging said inner bridging plates toward each other and into engagement with said stationary contacts and permitting rocking movement of said inner plates independently of said arm, means for providing a driving connection between said arm and said outer bridging plates and for resiliently urging said outer bridging plates toward each other and into engagement with said stationary contacts and permitting rocking movement of said outer bridging plates independently of said `arm and of said inner plates, each bridging plate being self-aligning and having three point support on said adjacent stationary contacts and on said portion extending into said aperture in said driving arm.

2. In combination, a plurality of stationary contacts uniformly spaced in an arc about an axis, an operating shaft extending along said axis, a driving arm extending radially from said shaft and having an aperture therethrough, a pair of conductive, inner bridging plates on opposite sides of the medial plane of said stationary contacts and of said arm, a pair of conductive, outer bridging plates on opposite sides of said arm and superposed 1over said inner bridgingplates, each bridging plate having a width greater than the space between adjacent stationary contacts to bridge said `adjacent .stationary contacts, each bridging plate having apportion extending into said aperture in said driving arm and said portions on said inner plates being in engagement and said portions on said outer plates being in engagement, whereby current can transfer from one of a pair of bridging plates to the other if a high resistance exists between said one bridging plate and a stationary Contact, said driving arm and said inner bridging plates having registering holes therethrough, means including at least one first pin extending through said registering holes and a spring circumjacent said iirst pin for providing a driving connection between said arm and said inner bridging plates and for resiliently urging said inner plates toward each other and into engagement with said stationary contacts and permitting rocking movement of said inner plates independently of said arm, said driving arm and said outer bridging plates having registering openings therethrough, and means including at least one second pin extending through said registering openings and a spring circumjacent said second pin for providing a driving connection between said arm and said outer plates and permit-ting rocking movement of said outer plates independently of said arm and of said inner plates, each bridging plate being self-aligning and having three point support on said adjacent stationary contacts and said portion extending into said aperture in said driving arm.

3. A movable contact assembly for a tap changer switch having a plurality of stationary contacts spaced in an arc about a central axis, an operating shaft extending along said axis, and a driving arm extending laterally from said shaft and generally in the plane of said stationary contacts, said driving arm having an aperture therethrough, said Contact assembly including a pair of conductive inner bridging plates on opposite sides of the plane of said stationary contacts and of said driving arm and a pair of conductive outer bridging plates on opposite sides of said plane and of said arm and superposed over said inner bridging plates, each bridging plate having a width greater than the space between adjacent stationary contacts to bridge said adjacent stationary contacts, said outer bridging plates engaging said stationary contacts through apertures in said inner bridging plates, means for providing a driving connection between said inner bridging plates and said driving arm and for resiliently urging said inner bridging plates toward each other and into engagement with said stationary contacts, means for providing a driving connection between said outer bridging plates and said driving arm and for resiliently urging said outer bridging plates toward each other and into engagement with said stationary contacts, both said means for providing a driving connection permitting said bridging plates to be movable independently of said arm and each said bridging plate being self-aligning and being supported at three points two of which are on said adjacent stationary contacts and the third of which is on the portion extending into said aperture in said arm, whereby mechanical forces imparted to said bridging plates are not transmitted to said driving arm.

4. A movable contact assembly for a tap changer switch having a plurality of sta-tionary contacts spaced in an arc about a central axis, an operating shaft extending along said axis, and a driving arm extending laterally from said shaft and generally in the plane of said stationary contacts, said driving arm having an aperture therethrough, said contact assembly including a pair of conductive inner bridging plates on opposite sides of the plane of said stationary contacts and of said driving arm and a pair of conductive outer bridging plates on opposite sides of the plane of said stationary contacts and of said driving arm and a pair of conductive outer bridging plates on opposite sides of said plane and of said arm and superposed over said inner bridging plates, each bridging plate having a width greater Ithan the space between adjacent stationary contacts to bridge said adjacent stationary contacts and each having two circumferentially spaced apart button contacts each of which engages one of said adjacent stationary contacts, each bridging plate having a portion ex- 'i tending into said aperture in said driving arm and said portions on said inner bridging plates being in engagement and said portions on said outer bridging plates being in engagement, each bridging plate being self-aligning and having three point support on said two button contacts thereon and said portion extending into said aperture in said driving arm, means for providing a driving connection between said inner bridging plates and said arm and for resiliently urging said first inner bridging plates toward each other and into engagement with said stationary contacts and permitting movement of said inner bridging vplates independent of said arm, and means for providing a driving connection between said outer bridging plates and said arm and for resiliently urging said outer bridging plates Vtoward each other and into engagement with said stationary contacts and permitting movement of outer Vbridging plates independent of said arm and of said inner bridging plates.

5. A movable contact assembly for a tap changer switch having a plurality of stationary contacts spaced in an arc about a central axis, an operating shaft extending along said axis, and a driving arm extending laterally from said shaft and generally in the plane of said stationary contacts, said driving arm having an aperture therethrough, said contact assembly including a pair of conductive inner bridging plates on opposite sides of the plane of said stationary contacts and of said driving arm and a pair of conductive outer bridging plates on opposite sides of said plane and of said arm and superposed over said inner bridging plates, each bridging plate having a width greater than the space between adjacent stationary contacts to bridge said adjacent stationary contacts and each having two circumterentialiy spaced apart button contacts each of which engages one of said adjacent stationary contacts, each inner bridging plate having a portion extending through said aperture in said driving arm end engaging said other inner bridging member to provide, conjointly wtih said button contacts thereon, three poi-nt support thereof, each outer bridging plate having a portion extending through said aperture in said driving arm and engaging said other outer bridging plate to provide, conjointly with said button contacts thereon, three point support thereof, said button contacts on said outer bridging plates extending through apertures in said inner bridging plates to`engage said stationary contacts, means including at least one tirst pin extending through registering apertures in said inner bridging plates and said driving arm and a spring surrounding said irst pin for resiliently urging said inner bridging plates into engagement with said stationary contacts and for providing a driving connection between said driving arm and said inner bridging plates, and means including at least one second pin extending through registering apertures in said inner bridging Vplates and said outer bridging plates and said driving arm and a spring surrounding said second pin for resiliently urging said outer bridging plates into engagement with said stationary contacts and for providing a driving connection between said driving arm and said outer bridging plates, said inner bridging plates being self-aligning and having independent rocking movement relative to said arm, said outer bridging plates being self-aligning and having independent rocking movement relative to said arm and to said inner bridging plates.

6. A movable contact assembly or a tap changer including an insulating panel having lan opening therein and having a plurality of stationary contacts mounted on said panel and uniformly spaced in an arc about a central axis through said opening, an operating shaft extending through said opening along said axis, and a driving arm extending laterally from said shaft and generally in the median plane of said stationary contacts; said driving arm having an aperture therethrough, said movable contact assembly including a pair of inner conductive members disposed on opposite sides of said median plane of t ti said stationary contacts and on opposite sides ot said driving arm and having width greater than the space between adjacent stationary contacts to bridge adjacent stationary contacts, each said inner member having two circumterentially spaced apart protuberances each of which engages one `of said adjacent stationary contacts bridged by said inner member and also having a portion radially inward from said protuberances extending into said aperture in said driving arm and engaging said other inner member to provide, conjointly with said protuberances, three point support for said inner member, means including at least one pin extending through registering apertures in said driving arm and both said inner members and a spring surrounding said pin for resi-liently urging said inner members toward each other and `said protuberances thereon into engagement with said stationary contacts and for providing a driving connection be ween said arm and said inner members and permitting rocking movement of said inner members independently oi said arm, a pair of outer conductive members disposed on opposite sides of said driving arm and superimposed over said inner members, each of said outer members having a width greater than the space between adjacent stationary contacts and having a pair of protuberances thereon each of which extends through an aperture in one of said inner members into engagement with one of said adjacent stationary contacts, each ot sai-d outer members having a portion radially inward from said protuberances thereon extending through said aperture in said driving arm and engaging the other outer member to provide, conjointly with said two protuberances, three point support for said outer members, and means including at least one pin extending through registering apertures in said arm and in said inner and said outer members and a spring surrounding said pin for resiliently urging said outer members toward each other and said protuberances thereon into engagement with said stationary contacts and for providing a driving connection between said arm and said outer members and permitting rocking movement of said outer members independently of said arm and of said inner members.

7. In combination, a plurality of stationary contacts spaced in an arc about an axis, an operating shaft extending along said axis, a driving arm extending radially from said shaft and having an aperture therethrough, two conductive bridging plates each of which has a width greater than the space between adjacent stationary contacts and is adapted to bridge adjacent stationary contacts, said bridging plates being positioned on opposite sides of the median plane of said stationary contacts and on opposite sides of said driving arm and engaging opposite sides of said stationary contacts, each of said bridging plates having a portion extending into said aperture in said driving arm and engaging the other bridging plate, whereby current can transfer from one bridging plate to the other if a high resistance exists between one bridging plate and a stationary Contact, means for providing a driving connection between said arm and said bridging plates and for resiliently urging said bridging plates toward each other and into engagement with said stationary contacts, said last-named means permitting movement of said bridging plates independently of said arm, each bridging plate being self-aligning and having three point support, two of said points being on said stationary contacts and the third being on said portion extending into said aperture in said arm.

8. In combination, a plurality of stationary contacts spaced in an arc about an axis, an operating shaft extending along said axis, a driving arm extending radially from said shaft and having an aperture therethrough, two conductive bridging plates each of which has a width greater than the space between adjacent stationary contacts and is adapted to bridge adjacent stationary contacts, said bridging plates being positioned on opposite sides of the median plane of said stationary contacts and on opposite sides of said driving arm and engaging opposite sides of said statitonary contacts, each of said bridging plates having a portion extending into said aperture in said driving arm and engaging the other bridging plate, whereby current can transfer from one bridging plate to the other if a high resistance exists between one bridging plate and a stationary contact, each bridging plate having a pair of circumferentially spaced apart protuberances thereon each of which is adapted to engage one of said adjacent stationary contacts bridged by said plate, said bridging plates and said arm having registering holes therethrough, and means including at least one pin extending through said registering holes and a spring circumjacent said pin for providing a driving connection between said arm and said bridging plates and for resiliently urging said bridging plates toward each other and into engagement with said stationary contacts, said last-named means permitting rocking movement of said bridging plates independently of said arm, each said bridging plate being self-aligning and having three point support on said two protuberances and said portion extending into said aperture in said driving arm. Y

9. In combination, a plurality of stationary contacts spaced in an arc about an axis, an operating shaft extending along said axis, a driving arrn extending radially from said shaft and having an aperture therethrough, two conductive bridging plates each of which has a width greater than the space between adjacent stationary contacts and is adapted to bridge adjacent stationary contacts, said bridging plates being positioned on opposite sides of the median plane of said stationary contacts and on opposite sides of said driving arm and engaging opposite sides of said stationary contacts, each of said bridging plates having a portion extending into said aperture in said driving arm and engaging the other bridging plate, whereby current can transfer from one bridging plate to the other if a high resistance exists between one bridging plate and a stationary contact, said bridging plates and said arrn having registering holes therethrough, and means including at least one pin extending through said register-ing holes and a spring circumjacent said pin for providing a driving connection between said arm and said bridging plates and for resiliently urging said bridging plates toward each other and into engagement with said stationary contacts, said last-named means permitting ro-cking movement ofk each bridging plate independently of the other and of said arrn and each bridging plate being self-aligning and having three point support on said adjacent stationary contacts and said portion extending into said aperture in said arrn.

References Cited by theExamner UNITED STATES PATENTS 2,682,643 6/54 Thias.

2,686,236 8/54 Hodtum 200-11 2,704,838 3/55 Macha et al 200-166 X 2,760,017 8/ 56 Stenersen 200-11 2,918,541 12/59 Waite 200e-11 2,921,996 1/ 60 Dulude 200-11 3,045,090 7/62 Wilson et al. 20G-1l X BERNARD A. GILHEANY, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,192,328 June Z9, 1965 Gordon A Wilson, Jr.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 4Z, after "connection" insert between co1umn 6, lines 68 and 69, strike out "and a pair of conductive outer bridging plates on opposite sides of said plane and of said arm"e Signed and sealed this 19th day of April 1966o (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. IN COMBINATION, A PLURALITY OF STATIONARY CONTACTS UNIFORMLY SPACED IN AN ARC ABOUT AN AXIS, AN OPERATING SHAFT EXTENDING ALONG SAID AXIS, A DRIVING ARM EXTENDING RADIALLY FROM SAID SHAFT AND HAVING AN APERTURE THERETHROUGH, A PAIR OF CONDUCTIVE, INNER BRIDGING PLATES ON OPPOSITE SIDES OF THE MEDIAL PLANE OF SAID STATIONARY CONTACTS AND OF SAID ARM, A PAIR OF CONDUCTIVE, OUTER BRIDGING PLATES ON OPPOSITE SIDES OF SAID ARM AND SUPERPOSED OVER SAID INNER BRIDGING PLATES, EACH BRIDGING PLATE HAVING A WIDTH GREATER THAN THE SPACE BETWEEN ADJACENT STATIONARY CONTACTS TO BRIDGE SAID ADJACENT STATIONARY CONTACTS, EACH BRIDGING PLATE HAVING A PORTION EXTENDING INTO SAID APERTURE IN SAID DRIVING ARM AND SAID PORTIONS ON SAID INNER PLATES BEING IN ENGAGEMENT AND SAID PORTIONS ON SAID OUTER PLATES BEING IN ENGAGEMENT, WHEREBY CURRENT CAN TRANSFER FROM ONE OF A PAIR OF BRIDGING PLATES TO THE OTHER IF A HIGH RESISTANCE EXISTS BETWEEN SAID ONE BRIDGING PLATE AND A STATIONARY CONTACT, MEANS FOR PROVIDING A DRIVING CONNECTION BETWEEN SAID ARM AND SAID INNER BRIDGING PLATES AND FOR RESILIENTLY URGING SAID INNER BRIDGING PLATES TOWARD EACH OTHER AND INTO ENGAGEMENT WITH SAID STATIONARY CONTACTS AND PERMITTING ROCKING MOVEMENT OF SAID INNER PLATES INDEPENDENTLY OF SAID ARM, MEANS FOR PROVIDING A DRIVING CONNECTION BETWEEN SAID ARM AND SAID OUTER BRIDGING PLATES AND FOR RESILIENTLY URGING SAID OUTER BRIDGING PLATES TOWARD EACH OTHER AND INTO ENGAGEMENT WITH SAID STATIONARY CONTACTS AND PERMITTING ROCKING MOVEMENT OF SAID OUTER BRIDGING PLATES INDEPENDENTLY OF SAID ARM AND OF SAID INNER PLATES, EACH BRIDGING PLATE BEING SELF-ALIGNING AND HAVING THREE POINT SUPPORT ON SAID ADJACENT STATIONARY CONTACTS AND ON SAID PORTION EXTENDING INTO SAID APERTURE IN SAID DRIVING ARM. 